Concerning electronic/electric apparatuses, reductions in size and weight have been progressing. This trend is attended by increases in current density and heating value of the apparatuses, and insulating materials used in the apparatuses have been required to have higher heat resistance. Meanwhile, epoxy resins, which are low in cost and high in adhesion properties, heat resistance, processability and the like, have been widely used as insulating material for electronic/electric apparatuses. Exemplary techniques for enhancing the heat resistance of epoxy resin include the introduction of a rigid structure, for example, a naphthalene skeleton, as described in Patent Document 1, and the application of a polyfunctional epoxy resin having three or more epoxy groups in its structure, as described in Patent Documents 2 and 3.
Similarly, the techniques for realizing stiffness and higher functionality has been investigated for their application to acid anhydrides used as a curing agent for epoxy resin. Examples of such investigation include methylnadic anhydride (MHAC-P, produced by Hitachi Chemical Co., Ltd.; acid anhydride equivalent: 178), methylhexahydrophthalic anhydride (HN-5500, produced by Hitachi Chemical Co., Ltd.; acid anhydride equivalent: 168), and methyltetrahydrophthalic anhydride (HN-2200, produced by Hitachi Chemical Co., Ltd.; acid anhydride equivalent: 166) which are described in Patent Document 2. In Examples 52 to 54 in the document, it is shown that a heat resistant temperature index increases with an increase in acid anhydride equivalent, in other words, with an increase in molecular weight. In addition, Patent Document 4 discloses an example of the use of methylcyclohexenetetracarboxylic dianhydride (which is tetrafunctional) as a curing agent. It is described in the document that in the exemplary case the glass transition temperature is elevated, as compared with the case where methylhexahydrophthalic anhydride is used as a curing agent.
Besides, Patent Document 5 discloses that when 2 to 10% by weight of triallyl cyanate is blended into an epoxy resin composition in which SMA (a copolymer of maleic anhydride and styrene) is used as a curing agent, the cured product shows a raised glass transition temperature. In the cases where an epoxy resin made polyfunctional and rendered stiff, an acid anhydride made polyfunctional, and/or an acid anhydride having a large molecular weight is used to enhance the heat resistance of insulating material, the varnish is raised in melting temperature and viscosity, leading to lowered moldability.
In addition, insulating materials are required to have high thermal conductivity, toughness and withstand voltage, and a technique for improving properties of insulating materials by blending organic and/or inorganic filler is searched for, as for example described in Patent Document 6. An addition of organic and/or inorganic fillers to a varnish causes an increase in melt viscosity. A lowering in moldability hampers formation of a fine and complicated insulation structure, and, therefore, promotes the generation of voids between wirings. Furthermore, the lowering in moldability makes difficult the process of deaerating the resin composition, and, therefore, promotes the remaining of bubbles in the insulating layer. The voids between wirings and the bubbles in the insulating layer damage the insulation reliability of electronic/electric apparatuses, and should therefore be avoided. Especially, in the case of casting varnishes for manufacturing insulating layers, anchoring layers, housings and the like by impregnation and/or casting process, enhanced heat resistance and lowered viscosity are requested.
Patent Document 7 discloses a cured resin product obtained by molding an alumina ceramic with a thermosetting resin composition containing a polyfunctional epoxy resin which has two or more epoxy groups in one molecule thereof, an acid anhydride curing agent, an inorganic filler, and a surfactant. The cured resin product has a safety factor of stress of not less than 7, a safety factor of strain of not less than 10 and an acceptable defect size of not less than 0.1 mm when cooled to room temperature, and has a thermal conductivity of 0.7 to 2.5 W/m·K.
Patent Document 8 discloses a liquid thermosetting resin composition containing (a) an imide ring-containing epoxy compound which is obtained by preliminarily putting an epoxy resin containing at least two epoxy groups in one molecule thereof and having an average molecular weight of not more than 1,000, a phenoxy resin, and a polymaleimide compound having at least two maleimide groups in one molecule thereof into reaction, (b) a polyfunctional vinyl monomer which has two or more acrylic groups, methacrylic groups or allyl groups in one molecule thereof, and (c) a liquid acid anhydride. In the document, however, there is no discussion in regard of lowering of viscosity of the epoxy resin composition or an improvement in crack resistance of the composition.